Transport across membranes

What affects the rate of diffusion?

- Temperature

- Concentration gradient

- Surface area

- Diffusion distance

- Size of molecule

- Stirring/moving of molecules

Define the 2 types of transport across membranes.

- Passive transport: doesn't require any external energy

- Active transport: requires energy in the form of the energy molecule ATP

Define diffusion

The net movement of particles from a region of high concentration to a region of lower concentration until evenly distributed.

What are the 2 types of diffusion?

- Simple diffusion

- Facilitated diffusion

Define simple diffusion

Allows small, lipid-soluble, uncharged molecules, e.g. Oxygen, Carbon dioxide, to directly cross the phospholipid bilayer.

Define facilitated diffusion

Uses channel proteins to allow small, water soluble molecules (mainly ions or polar) e.g. Na+ ions, H+ ions to cross the phospholipid bilayer. And uses carrier proteins to allow larger, water-soluble (mainly ions or polar) e.g. glucose, amino acids to cross the phospholipid bilayer.

Define active transport

The movement of particles into or out of a cell from a region of lower concentration to a region of higher concentration (against its concentration gradient) using ATP and carrier proteins.

Recall how active transport takes place across a membrane.

1) The molecule or ion to be transported across the membrane binds to specific carrier proteins on one side of the membrane (at binding site).

2) On the inside of the membrane, ATP binds to the carrier protein, causing it to hydrolyse into ADP + Pi (releasing the ADP).

3) This causes the carrier protein to change shape and open to the opposite side of the membrane (releasing the molecule or ion to the opposite side).

4) The inorganic phosphate is released from the carrier protein causing it to revert back to its original shape.

5) This process can then be repeated.